Avengers worked because we had each hero built up in at least one film before they teamed up. So we got to know the characters.
I don't think this is an entirely true statement. In general, Marvel does a good job introducing characters and letting us get to know them before throwing them into other things or the big Avengers movies. But I don't think you can credit this for the success of the first Avengers movie. Although we *saw* Hawkeye in Thor, we were not introduced to the character until Avengers. Hawkeye's cameo in Thor is not all that different from the cameos of the Flash and Aquaman in BvS. And while we were introduced to Black Widow in Iron Man 2, we were not really introduced to the
character much. Even the Hulk is kind of soft-rebooted in Avengers. The Avengers is anchored in the characters of Tony Stark and Steve Rogers, but it spends time building the other characters.
I think it is a true statement that the Avengers movie is a logical out growth of the prior movies and a very good payoff for Iron Man, Thor, and First Avenger. But it is a very strong stand alone movie in its own right. And the three prior movies were also strong stand alone movies in their own right. I think this shows when you consider that I believe Marvel word of mouth is a lot stronger than DCEU word of mouth. There are a lot of Marvel fans who don't like all of the movies but are willing to take a chance on them. There are a lot of Marvel fans who will recommend people watch a Marvel movie even if the person they are recommending the movie to didn't like previous Marvel movies.
Even if you didn't like Thor, you might like Guardians. If you didn't like Guardians, try Winter Soldier. Its okay to not like Winter Soldier, try Ant-Man. Even if you didn't like any of them, try Dr. Strange or Ragnarok, maybe you'll like them. These are things Marvel fans will tell people about upcoming Marvel movies. But nobody really says this about the DCEU movies. They don't stand alone. If you liked one of them, you still might not like the others. But if you didn't like MoS, odds are really good you're not going to like BvS either. And I while I think more people will like JL than BvS, even I wouldn't recommend JL strongly to someone that doesn't like the current DCEU movies.
This is the real strength of the MCU in my opinion. They didn't have access to the obvious franchise building properties. They couldn't use X-Men. They couldn't use Spiderman. They couldn't use the FF. So they had to start from scratch, and they had a weird hodgepodge group to try to build around. Iron Man, Thor, and Captain America are all very different characters, with very different sensibilities. Each had to exist in a very different story telling context. That diversity became a huge strength for Marvel: it bought them the opportunity to branch out and experiment. X-Men after X-Men movie doesn't buy you the opportunity to launch a Guardians, or an Ant Man, or a Dr. Strange.
Even if you didn't like *any* Marvel movie or have never seen a single one before I would still recommend you try Black Panther coming up. I am willing to stick my neck out and ask someone, even a non-Marvel fan, to give it a shot. I don't think people are sticking their necks out for the DCEU, even the people who like those movies, and that's ultimately what is costing them box office.
JL looks like it will end up making about as much money as Dr. Strange. Now, you can look at that and say that since Dr. Strange cost much less money to make it is the more successful movie (Dr Strange cost about half as much). But most people don't know that. Most people only know how long the lines are for both movies, how many of their friends saw those movies, and how much people are talking about those movies. The same number of people are ultimately going to watch those two movies in the theaters, and yet the buzz is that JL is a failure and Dr. Strange is a success. And yet they both captured a similar number of eyeballs. That's very important to take note of. Forget the budget: most of us don't know the budget. But we had a certain expectation for Strange: it was a quirky journey to an unexplored area of the MCU, and it delivered on that. That's what makes it a success in the eyes of the fans and the movie going audience, not the numbers. We expected great things from JL, and even the average uninformed movie going audience member probably expected a lot going into that movie given the build up to it. The movie didn't deliver, so that makes it not a success.
Money isn't everything. Suicide Squad was profitable. By some analyses, Suicide Squad was more profitable than Winter Soldier. But there's no question which movie was the more successful one.